1. Field of the Invention
Methane production by anaerobic digestion has been widely practiced, particularly with respect to digestion of sewage sludge organic waste. In recent times, the world-wide energy shortage has furthered consideration and improvement of such non-fossil sources of energy. This invention relates to a process for improved methane production from and beneficiation of anaerobic digestion comprising anaerobic digestion of mixtures of plant material and organic waste. The process may be carried out under mesophilic or thermophilic temperatures for detention times in excess of about five days. Particularly preferred is for the plant material to consist of terrestrial and aquatic plant materials. Under steady state anaerobic digestion, the mixed feed blend of plant material and organic waste results in synergistic action providing higher methane yields and production rates than those that result from the anaerobic digestion of the individual feed components separately.
2. Description of the Prior Art
The production of methane gas by anaerobic digestion of various organic wastes has been known. There have been continuous efforts to improve methane yield resulting from anaerobic digestion. Most of the prior attempts to increase methane yield have been centered around anaerobic digestion as practiced in municipal waste treatment plants as exemplified by U.S. Pat. Nos. 3,640,846, teaching addition of coal; 3,981,800, teaching pressurized digestion; and 4,022,665, teaching two phase digestion of sewage sludge. Other attempts to improve the production rate and yield of methane by anaerobic digestion have related to improved anaerobic digestion by utilization of liberated enzymes of the biomass for contribution to more efficient digestion as taught by U.S. Pat. No. 3,994,780. The U.S. Pat. No. 3,994,780 teaches the applicability of its process to a wide variety of organic feeds, but does not suggest the synergistically improved methane production by anaerobic digestion of a mixture of plant material and organic waste. The anaerobic digestion of terrestrial plant material to produce methane has been recognized as exemplified by D. L. Klass and S. Ghosh, "Methane Production by Anaerobic Digestion of Bermuda Grass", presented at symposium on Biomass as a Non-fossil Fuel Source, ACS/Chem. Soc. of Japan Joint Chemical Congress, Honolulu, Hawaii, Apr. 1-6, 1979. Likewise, the anaerobic digestion of aquatic plant material to produce methane has been recognized as exemplified by R. P. Lecuyer and J. H. Marten, "An Economic Assessment of Fuel Gas from Water Hyacinths", Symposium Papers, Clean Fuels from Biomass, Sewage, Urban Refuse, Agricultural Wastes, Orlando, Fla., Jan. 27-30, 1976. Again, the synergism resulting in improved methane production by anaerobic digestion of a mixture of plant material and organic waste is not suggested. Stimulation of methane production in anaerobic waste treatment by metal cations has been recognized as has the problem of toxicity in methane producing anaerobic systems as exemplified by I. J. Kugelman and K. K. Chin, "Toxicity, Synergism and Antagonism in Anaerobic Waste Treatment Processes", Anaerobic Biological Treatment Processes, Advances in Chemistry, Series 105 (1971). While recognizing the sensitivity of methane production in anaerobic waste treatment processes, there is no suggestion of the synergism of anaerobic digestion of a mixture of plant material and organic waste in methane production.